The present invention relates to FM (frequency modulation) demodulator. More particularly, the present invention relates to a FM demodulator in which the use of inductors for filtering is avoided an the output signal is substantially independent of fading. This independence is particularly useful in mobile receiver applications in which the strength of an input signal can vary quite rapidly.
It is known to demodulate frequency-modulated signals by a method which involves sampling in some way. However, a disadvantage of this method is that when the signal has been mixed down to audio frequencies, the sampling produces harmonics which lie in the audio band and cause distortion.
There also exists a number of proposals for demodulating FM signals which proposals do not involve sampling. British Patent Specification No. 1,530,602 discloses a demodulator comprising first and second parallel arranged signal paths whose inputs are connected to receive an incoming FM signal and whose outputs are connected to a difference amplifier. Each of the first and second signal paths comprises a first mixer having an output connected to a low pass filter whose output is in turn connected via an automatic level control (ALC) amplifier to one input of a second mixer the output of which is connected to a respective input of the difference amplifier. A first signal input to the first mixer in each path is the incoming FM signal. A second signal input of the first mixer in the second path is a reference signal and the second signal input of the first mixer in the first path is the same reference signal which has been phase shifted by 90.degree.. The outputs of the first mixers are the sum and difference components of mixing. The low pass filter in each path passes the difference components, namely a cos .delta. .omega.t in the first path and a sin .rarw..omega.t in the second path, where 2a is the amplitude of the input signal, .delta..omega. is the angular frequency difference component and the amplitude of a signal from a local oscillator is unity. The ALC amplifiers enable the signals processed thereafter to have a standard level of signal with which to work and enable the receiver to have a large working range signal.
The output signals from the ALC amplifiers are supplied to a first signal input of the second mixer in its respective signal path and to respective differentiators which differentiate their respective signals to give terms with amplitude/frequency slope and a quadrature phase shift. The output of the differentiator whose input is connected to the first path is connected as a second signal input to the second mixer in the second path; conversely, the output of the other differentiator is connected as a second signal input to the second mixer of the first path. The signals applied to the inputs of the difference amplifier comprise a.sup.2 .delta..omega. cos.sup.2 .delta..omega.t from the first path and -a.sup.2 .delta..omega. sin.sup.2 .delta..omega.t from the second path and the output from the difference amplifier is a.sup.2 .delta..omega.. Since this output comprises a power term, namely a.sup.2, then the output is dependent on the amplitude of the input signal which may vary considerably and rapidly if the demodulator is part of a mobile receiver.
Also known is a sine-cosine frequency tracker which is disclosed in British Patent Specification No. 1,363,396. The frequency tracker includes a frequency discriminator which is very similar to the demodulator of British Patent Specification No. 1,530,602 but in which there are no ALC amplifiers. The output from a difference amplifier is A.sup.2 .omega. where A is the amplitude of the input signal and .omega. is the difference signal between the incoming signal and a reference derived from a sweep oscillator. In the case of the tracking system, the A.sup.2 .omega. term is required in order to provide a control signal for the sweep oscillator.
U.S. Pat. No. 3,568,067 also discloses a frequency discriminator which is very similar to that disclosed in British Patent Specification No. 1,363,396. This too produces an output which is amplitude dependent.